The present invention relates to a coating solution used for forming a silica-based coating film on various substrate surfaces of semiconductor, glass, metal, ceramic and the like.
It is a prominent trend in recent years along with the rapid progress in the manufacturing technology of semiconductor devices or, in particular, VLSIs that the demand for technology upgrading is increasing in multi-layer wiring processes in order to obtain devices of high degree of integration, high operating velocity and multiplicity of performance.
As is known, the manufacturing process of VLSIs involves forming of wiring patterns and insulating layers on the substrate surface and it is usually unavoidable that a difference in levels is produced on the surface by such a treatment. When a wiring pattern is formed on such a surface having a level difference, it is natural that the accuracy of the wiring pattern can no longer be high enough thereby disadvantageously limiting the fineness of wiring. Accordingly, such a surface of substrate having a level difference must be subjected to a smoothing treatment prior to the subsequent patterning treatment.
In the prior art, the smoothing treatment above mentioned is performed by coating the substrate surface with a coating solution prepared by dissolving a silanol compound in an organic solvent, which is mainly an alcohol, to fill the recessed areas on the substrate surface having a level difference and further to overcoat the whole surface followed by a heat treatment or baking treatment so that a silica-based coating layer for smoothing is formed by the silanol condensation of the silanol compound. Such a method is usually called a spin-on-glass (SOG) coating method and widely practiced in the semiconductor technology. This method is truly very reliable because the smoothing coating layer thus formed is completely inorganic. On the other hand, the silica-based coating layer unavoidably has some brittleness so that cracks are sometimes formed in the coating layer, especially, when the coating layer has a thickness sufficiently large to completely fill the recessed areas and smooth the substrate surface having a level difference. Therefore, it is sometimes practiced that the spin-on-glass coating method is combined with a chemical vapor deposition method so as to form the smoothing coating layer in two different ways successively.
Several proposals have been made to develop an improved coating solution useful in the spin-on-glass coating method from which a coating layer of a relatively large thickness can be obtained without the danger of crack formation including an organic solution of a ladder polymer-type silicone. Coating solutions of this type, however, have problems from the standpoint of practical applications because the coating layer obtained therefrom has a relatively low heat resistance and decomposes at a temperature of 300.degree. to 400.degree. C. in addition to the poor moisture resistance.
Besides, the so-called biased sputtering method is sometimes undertaken to effect smoothing of the substrate surface having a level difference in minute areas, in which the substrate surface is lightly bombarded with charged particles so as not to leave any residual gas such as hydrogen, oxygen, nitrogen and the like in the substrate during formation of a coating layer. This method, however, is defective because the substrate surface is more or less damaged in the course of deposition of the coating layer.